import numpy as np
import pandas as pd

a = pd.Series([1,2,3,4], index=['a','b','c','d'])
b = pd.Series([1,2,3,4,9], index=['b','e','a','c','d'])

e = a + b
print(e.isnull())
print(e.notnull())
print(e.dropna())
print(e.fillna(0))
print(e.fillna(e.mean()))
